The present invention relates to a power supply device for regulating and supplying a voltage generated by a DC power supply to an outer circuit. The power supply device has a capability for controlling an output voltage to prevent circuit components from being damaged upon occurrence of a momentary output voltage interruption or short circuit.
In recent years, fast-paced efforts have been made for developing small-size, lightweight, and low-power-consumption electronic devices such as magnetic recording and reproducing devices with combined cameras. For lower power consumption, a switching power supply is indispensable to such electronic devices. It is primarily customary for the switching power supply to use transistors in performing its switching operation.
Electronic devices having dry batteries such as nickel-cadmium (NiCd) cells or nickel-hydrogen (NiH) cells as a power source are generally widely used in the art. Since these dry batteries have output voltages in a relatively low range, the voltages that can be supplied from these cells may not match voltages that are used in electronic devices. Therefore, a voltage converting device called a DC/DC converter is used to convert the output voltage of a dry battery for stably supplying a necessary power supply voltage to a desired electronic circuit.
If an abrupt large current flows from a switching power supply due to a short circuit in the load connected to the output stage of the switching power supply, then the fuse of an electronic device incorporating the switching power supply may possibly melt, or transistors or other components of the electronic device may be broken. According to one approach to solving this problem, a short-circuit detecting comparator is incorporated in the IC (Integrated Circuit) of a switching power supply. When the output voltage of the switching power supply exceeds a certain preset voltage, a timer latch circuit is activated. The output voltage of the switching power supply is cut off if it remains continuously in excess of the preset voltage for a period of time set by the timer latch circuit.
Some conventional DC/DC converters have a switching element in the form of a bipolar transistor The amount of current that flows in a DC/DC converter is limited by the base current of a bipolar transistor. Consequently, it is uncommon for the fuse of an electronic circuit incorporating a DC/DC converter to melt while the timer latch is monitoring the output voltage for a period of time set thereby.
Recently available DC/DC converters employ a switching element in the form of an N-channel MOS-FET (Metal Oxide Semiconductor-Field Effect Transistor), which is selectively turned on and off depending a switching pulse applied thereto.
A conventional boosting DC/DC converter power supply device will be described below. When an input voltage is applied from a dry battery, a voltage converting circuit converts the input voltage into an output voltage Vo, which is supplied to a load circuit. A PWM (Pulse Width Modulation) control circuit generates a PWM signal for turning on and off a switching transistor of the voltage converting circuit. The output current from the switching transistor is rectified to produce the output voltage Vo.
The voltage converting circuit will be described below. In the voltage converting circuit, an FET has a drain connected to a power supply terminal through a choke coil and a source connected to ground. The FET has a gate connected to a pulse input terminal for being supplied with a switching pulse from the PWM control circuit.
The junction between the FET and the choke coil is connected to the anode of a diode (Schottky diode). A capacitor has a terminal connected to the cathode of the diode and another terminal connected to ground.
When the FET is switched off by the switching pulse, a voltage higher than the voltage at the power supply terminal is produced at the junction between the choke coil and the diode due to the excited energy discharged from the choke coil, charging the capacitor. The voltage at the junction gradually drops and reaches a level substantially equal to the ground voltage when the FET is turned on again. Therefore, a voltage change in response to the switching pulse appears at the anode of the diode. The voltage change is rectified by the diode into a voltage higher than the input voltage. The rectified voltage is smoothed by the capacitor, and the smoothed voltage is supplied from an output terminal to the load.
Japanese Patent Laid-open No. Hei 11-164548 discloses a power supply device for preventing an overcurrent from flowing through a switching element when an input AC voltage abruptly increases. According to the disclosed power supply device, when the input AC voltage is momentarily interrupted, a comparator does not generate a pulse because it is not supplied with a voltage from a rectifying circuit during the momentary interruption of the input AC voltage. At this time, a pulse detecting circuit cannot detect a pulse and outputs a breaking signal. Since the pulse detecting circuit outputs the breaking signal with a delay of about 20 msec., it continuously outputs the breaking signal for about 20 msec. even after the momentary interruption of the input AC voltage is recovered. When a drive pulse generating circuit receives the breaking signal from the pulse detecting circuit, it stops generating a drive pulse, turning off the switching element.
Japanese Patent Laid-open No. Sho 63-77375 reveals a starter circuit for a DC/DC converter, the starting circuit being capable of preventing an output voltage of the DC/DC converter from abruptly rising due to a main power supply interruption. When a voltage supplied from the main power supply to the primary side of the DC/DC converter exceeds a reference voltage, a soft starter circuit for the DC/DC converter is energized again.
The applicant of the present application has applied for a patent on a power supply device as disclosed in Japanese Patent Application No. 2003-281968. According to the disclosed power supply device, even when a voltage supplied from a dry battery is temporarily interrupted, an upper limit (MAX-Duty) for a PWM signal is forcibly lowered to prevent an excessive voltage from being applied to the load, thereby preventing a device connected to the load from being broken and also preventing a PWM control IC to which the output voltage is applied as a feedback signal from being broken.
When a short circuit occurs in the circuit employing the FET of the above DC/DC converter, since the short circuit occurs with the on resistance (several hundred milliohms) of the FET, a large current flows at the instant of the short circuit. Therefore, before the short-circuit detecting mechanism of the IC operates, the fuse may melt, or the FET may be destroyed.
The above problem arises in the event of a short circuit occurring in certain adverse user environments such as moisture condensation. Even through the problem can be solved by improving such adverse user environments, once the fuse melts, the power supply device needs to be taken care of by the manufacturer's repair services.
The power supply device disclosed in Japanese Patent Laid-open No. Hei 11-164548 turns off the switching element in the event of an input AC voltage interruption. The starter circuit disclosed in Japanese Patent Laid-open No. Sho 63-77375 detects that the input voltage applied to the primary side of the DC/DC converter exceeds a prescribed level and energizes again the soft starter circuit for the DC/DC converter.
A power supply device according to the present invention is not arranged to stop the output only when the input voltage is interrupted as disclosed in Japanese Patent Laid-open No. Hei 11-164548, but detects a short circuit in various output voltage patterns and stops a PWM signal. The power supply device according to the present invention does not energize a soft starter circuit again unlike the starter circuit disclosed in Japanese Patent Laid-open No. Sho 63-77375.